PRIME NUMBER THEOREM

Transcription

1 PRIME NUMBER THEOREM RYAN LIU Abstract. Prime numbers have always been seen as the building blocks of all integers, but their behavior and distribution are often puzzling. The prime number theorem gives an estimate for how many prime numbers there are under any given positive number. By using complex analysis, we are able to find a function π(x) that for any input will give us approximately the number of prime numbers less than the input. Contents. The Prime Number Theorem 2. The Zeta Function 2 3. The Main Lemma and its Application 5 4. Proof of the Main Lemma 8 5. Acknowledgements 6. References. The Prime Number Theorem A prime number is an interger 2 which is divisible only by itself and. Thus the prime numbers start with the sequence 2,3,5,7,,3,7,9,... Since these numbers are indivisible but anything other than itself and, we can see them as the building blocks of all other numbers. In fact, the Fundamental Theorem of Arithmetic tells us that every natural number can be factored into a unique set of primes. It would therefore be beneficial for us to find out these prime numbers, or how they are distributed amongst the natural numbers. By looking at the first few prime numbers, it is hard to believe that there could be any pattern among the prime numbers, but using complex analysis, we are able to come out with a very simple representation of the distribution of prime numbers. In order to do so, we must first define a function π(x) which will output the number of primes x. The prime number theorem then describes how π(x) behaves for real numbers x. Theorem.. (Prime Number Theorem) We have π(x) x log x. Date: July 2, 22.

2 2 RYAN LIU Somehow, we are able to relate the distribution of prime numbers to a simple x fraction, logx. To understand how this makes sense, we will need to start from a more basic function, the zeta function. 2. The Zeta Function Let s be a complex variable. For Re(s) > the series n s, n= converges absolutely, and uniformly for Re(s) + δ, with any δ >. This series is called the zeta function. One sees this convergence by estimating n s n +δ, and by using the integral test on the real series /n +δ, which has positive terms. The Reimann Zeta Function has been used in algebraic number theory because of its usefulness in dealing with prime numbers. We will see its power in the next proposition. Proposition 2.. The product p ( p s ), converges absolutely for Re(s) >, and uniformly for Re(s) + δ with δ >, and we have ζ(s) = ( ) p s. p Proof. The product converges since each term is less than and thus In the same region Re(s) + δ, we can use the geometric series estimate to conclude that ( ) p s = + p s + p 2s + p 3s +... = E p(s). Using a basic fact from elementary number theory that every positive integer has unique factorization into primes, up to the order of the factors, we conclude that in the product of the terms E p (s) for all primes p, the expression /(n s ) will occur exactly once, thus giving the series for the zeta functino in the region Re(s) >. This concludes (Euler s) proof. The product p ( p s ), is called the Euler product. The representation of the zeta function as such a product shows that ζ(s) for Re(s) >. Since this product relates all primes

3 PRIME NUMBER THEOREM 3 to all natural numbers, if we are able to study this function, we can conclude the distribution of primes amongst the natural numbers. In order to make this function more usable, we want to extend it to all Re(s) >, however we also have to make sure that the resulting function behaves in a way we want it to so we can analyze it further. Theorem 2.2. The function ζ(s) s, extends to a holomorphic function on the region Re(s) > Proof. For Re(s) >, we have ζ(s) s = = n s n= n+ n= n x s dx ( n s x s We estimate each term in the sum by using the relations f(b) f(a) = b a therefore each term is estimated as follows: n+ ( n s ) x s dx max n ) dx. f (t)dt and so f(b) f(a) max f (t) b a, a t b n x n+ n s x s max s x s+ s n Re(s)+. Thus the sum of the terms coverge absolutely and uniformly for Re(s) > δ. This concludes the proof of the theorem We note that the zeta function has a certain symmetry about the x-axis, namely ζ(s) = ζ(s). This is immediate from the Euler product and the series expansion of Theorem 2.2. It follows that if s is a complex number where ζ has a zero of order m (which may be a pole, in which case m is negative), then the complex conjugate s is a complex number where ζ has a zero of the same order m. We now define ϕ(x) = p x log p and Φ(s) = p log p p s for Re(s) >. We will now try to use log because it allows us to turn the Euler product into sums which may be more manageable. The sum defining Φ(s) converges uniformly and absolutely for Re(s) + δ,

4 4 RYAN LIU by the same argument as for the sum defining the zeta function. We merely use the fact that given ɛ > log n n ɛ for all n n (ɛ) Theorem 2.3. The function Φ is meromorphic for Re(s) > 2. Furthermore, for Re(s), we have ζ(s) and has no poles for Re(s) Φ(s) s, Proof. For Re(s) >, The Euler product shows that ζ(s). derivative of log ζ(s) we get ζ ζ(s) = p log p p s. Using the geometric series we get the expansion so p s = p s /p s = p s (2.4) ζ ζ(s) = Φ(s) + p ( + p s + p 2s + = p s + p 2s +, ) h p (s) where h p (s) C log p p 2s, By taking the for some constant C. But the series (log n)/n 2s converges absolutely and uniformly for Re(s) 2 + δ with δ >, so Theorem 2.2 and Equation 2.4 imply that Φ is meromorphic for Re(s) > 2, and has a pole at s = and at the zeros of ζ, but no other poles in this region. There remains only to be proved that ζ has no zero on the line Re(s) =. Suppose ζ has a zero of order m at s = + ib with b. Let n be the order of the zero at s = + 2ib. Then m, n by Theorem 2.2. From Equation 2.4 we get lim ɛφ( + iɛ) =, lim ɛ ɛφ( + ɛ ± ib) = m, ɛ ɛφ( + ɛ ± 2ib) = n. lim ɛ Indeed, if a meromorphic function f(z) has the factorization then f(z) = (z z ) k h(z) with h(z ), f f(z) = k + holomorphic terms at z, z z whence our three limits follow immediatley from the definitions and the pole of the zeta function at, with residue.

5 PRIME NUMBER THEOREM 5 Now the following identity is trivially verified: Φ(s + 2ib) + Φ(s 2ib) + 4Φ(s + ib) + 4Φ(s ib) + 6Φ(s) = p log p p 2s (pib/2 + p ib/2 ) 4 For s = + ɛ > we see that the above expression is. using our limits, we conclude that 2n 8m + 6, whence m =. This concludes the proof of Theorem.4. Proposition 2.5. For Re(s) > we have Φ(s) = s ϕ(x) dx xs+ Proof. To prove this, compute the integral on the right between successive prime numbers, where ϕ is constant. Then sum by parts. 3. The Main Lemma and its Application Theorem 3.. (Chebyshev) We have ϕ(x) = O(x). Proof. Let n be a positive integer. Then 2 2n = ( + ) 2n = ( ) ( ) 2n 2n j n j hence we get the equality n<p 2n ϕ(2n) ϕ(n) 2n log 2. p = e ϕ(2n) ϕ(n), But if x increases by, then ϕ(x) increases by at most log(x + ), which is O(log x). Hence there is a constant C > log 2 such that for all x x (C) we have ϕ(x) ϕ(x/2) Cx. We apply this inequality in succession to x, x/2, x/2 2,, x/2 r and sum. This yields which proves the theorem. ϕ(x) 2Cx + O(),

6 6 RYAN LIU We shall now state the main lemma, which constitutes the delicate part of the proof. Let f be a function defined on the real numbers, and assume for simplicity that f is bounded, and piecewise continuous. What we prove will hold under much less restrictive condition: instead of piecewise continuous, it would suffice to assume that the integral b a f(t) dt, exists for every pair of numbers a, b. transform g defined by g(z) = We shall associate to f the Laplace f(t)e zt dt for Re(z) >. Lemma 3.2. (Main Lemma) Let f be bounded, piecewise continuous on the reals. Let g(z) be defined by g(z) = f(t)e zt dt for Re(z) > If g extends to an analytic function for Re(z), then exists and is equal to g() f(t) dt The main lemma will be proved in the next section, for now we will apply the main lemma to prove Lemma 3.3 Lemma 3.3. The integral converges Proof. Let ϕ(x) x) x 2 f(t) = ϕ(e t )e t = ϕ(et ) e t e t. f is certainly piecewise continuous, and is bounded by Theorem 2.. Making the substitution x = e t in the desired integral, dx = e t dt, we see that ϕ(x) x x 2 dx = f(t) dt. It suffices to prove that the integral on the right converges. By the main lemma, it suffices to prove that the Laplace transform of f is analytic for Re(z), so we have to compute this Laplace transform. We claim that in this case, Φ(z + ) g(z) = z + z. Once we have proved this formula, we can apply Theorem.3 to conclude that g is analytic for Re(z), thus concluding the proof of Lemma 2.3. Now to compute g(z), we use the integral formula of Proposition.4. By this formula, we obtain Φ(s) s s = dx, ϕ(x) x x s+ dx.

7 PRIME NUMBER THEOREM 7 By substituting z + for s we get Φ(z + q) z + z = = = ϕ(x) x x s+ dx ϕ(e t ) e t e zt e t dt e 2t f(t)e zt dt. This gives us the Laplace transform of f, and concludes the proof of Lemma 2.3. Let f and f 2 be functions defined for all x x, for some x. We say that f is asymptotic to f 2, and write Theorem 3.4. We have ϕ(x) x f f 2 if and only if lim x f (x)/f 2 (x) = Proof. The assertion of the theorem is logically equivalent to the comibnation of teh following two assertions: Given λ >, the set of x such that ϕ(x)/x λ x is bounded; Given < λ <, the set of x such that ϕ(x) λ x is bounded. Let us prove the first. Suppose the first assertion is false. Then there is some λ > such that for arbitrarily large x we have ϕ(x)/x λ. Since ϕ is monotone and increasing, we get for such x: λx x ϕ(t) t t 2 dt λx x λx t λ λ t t 2 dt = t 2 dt >. The number on the far right is independent of x. Since there are arbitrarily large x satisfying the above inequality, it follows that the integral of Lemma 2.3 does not converge, a contradiction. So the first assertion is proved. The second assertion is proved in the same way. Theorem 3.5. (Prime Number Theorem) We have π(x) x log x. Proof. We have and given ɛ >, ϕ(x) ϕ(x) = p x x ɛ p x log p p x log p x ɛ p x log x = π(x) log x; ( ɛ) log x = ( ɛ) log x π(x) + O(x ɛ ). Using Theorem 2.4 that ϕ(x) x concludes the proof of the prime number theorem

8 8 RYAN LIU 4. Proof of the Main Lemma We recall the main lemma Let f be bounded, piecewise continuous on the reals. Let g(z) = f(t)e zt dt for Re(z) > If g extends to an analytic function for Re(z), then Proof. For T > define f(t) dt exists and is equal to g() g T (z) = T f(t)e zt dt. g T is an entire function, as follows at once by differentiating under the integral sign. We have to show that lim g T () = g(). T Let δ > and let C be the path consisting of the line segment Re(z) = δ and the arc of circle z = R and Re(z) δ, By our assumption that g extends to an analytic function for Re(z), we can take δ small enough so that g is analytic on the region bounded by C, and on its boundary. Then g() g T () = (g(z) g T (z))e T z ( + z2 2πi C R 2 )dz z = H T (z) dz, 2πi C where H T (z) abbreviates the expression under the integral sign. Let B be a bound for f, that is f(t) B for all t. In order to prove g() g T () < ɛ, we will split up the path C into C and C + Let C + be the semicircle z = R and Re(z). Then we can prove (4.) H T (z) dz 2πi 2B C R + by first noting that for Re(z) > we have g(z) g T (z) = f(t)e zt dt B e zt dt T T = B Re(z) e Re(z)T and for z = R, ( ) et z + z2 R 2 z = R ere(z)t z + z 2 Re(z) R = ere(z)t R R 2.

9 PRIME NUMBER THEOREM 9 Taking the product of the last two estimates and multiplying by the length of the semicircles gives the bound for the integral over the semicircle, and proves the claim Now let C be the part of the path C with Re(z) <. We wish to estimate ( ) (g(z) g T (z)) e T z + z2 dz 2πi R 2 z C Now we estimate separately the expression under the integral with g and g T. We want to show that ) (4.2) g T (z)e ( T z + z2 dz 2πi C R 2 z B R. we do this by letting S be the semicircle with z = R and Re(z) <. Since g T is entire, we can replace C by S in the integral without changing the value of the integral, because the integrand has no pole to the left of the y-axis. Now we estimate the expression under the integral sign on S. We have g T (z) = T f(t)e zt dt B T B e Re(z)T Re(z) e Re(z)T dt For the other factor we use the same estimate as previously. We take the product of the two estimates, and multiply by the length of the semicircle to give the desired bound in (3.2). Third, we claim that ) (4.3) g(z)e ( T z + z2 dz C R 2 z as T We can write the expression under the integral sign as ( ) g(z)e T z + z2 R 2 z = h(z)et z where h(z) is independent of T. Given any compact subset K of the region defined by Re(z) <, we note that e T z rapidly uniformly for z K, as T The word rapidly means that the expression divided y any power T N also tends to uniformly for z in K, as T. From this our claim (3.3) follows easily We may now prove the main lemma. We have f(t) dt = lim T g T () if this limit exists. But given ɛ, pick R so large that 2B/R < ɛ. Then by (3.3), pick T so large that

10 RYAN LIU ) g(z)e ( T z + z2 dz C R 2 z < ɛ Then by (3.), (3.2), and (3.3) we get g() g T () < 3ɛ. This means that the equation converges, proving the main lemma. This concludes the proof of the prime number theorem. 5. Acknowledgements I would like to thank my mentor, Mohammad Abbas Rezaei, for introducing me to and helping me read through Lang s book, as well providing comments and suggestions on the initial drafts of this manuscript. I would like to thank Peter May for reading through the manuscript, as well as for his role in putting on the VIGRE program at the University of Chicago, which allowed me to pursue my interest in number theory. 6. References [] Serge Lang, Complex Analysis, Springer-Verlag, New York, 993